Johnny Dixon #3 : The Spell of the Sorcerer's Skull by John Bellairs

Trade Paperback.

A bookish boy searches for his missing best friend in this spooky tale by the author of The House with a Clock in Its Walls
 
On a country lane in snowbound 1950s New Hampshire, a car goes skidding off the road. Professor Childermass and Johnny Dixon escape unscathed, but their car is stuck, and they are forced to walk into town. Johnny doesn’t mind. A curious young man, he has fun anytime the professor takes him out, because he’s treated like an adult. Together they’ve gotten into all sorts of supernatural scrapes, and this winter night, they’ll face their toughest challenge yet.
 
When Childermass suddenly vanishes, Johnny is the only one who can find him. The mystery is linked to a tiny skull taken from a child’s dollhouse, which seems to have powers too terrible to guess at. With the help of a crusty old Irish priest, Johnny chases the clues to his friend’s disappearance all the way to the rocky coast of Maine, where something evil hungers for revenge.
 
From the author of the series featuring Lewis Barnavelt and Anthony Monday, the Johnny Dixon novels are charmingly old-school and shot through with suspense, and The Spell of the Sorcerer’s Skull may be the most chilling of them all.